1. The Field of the Invention
This present invention relates to the field of telecommunications. More particularly, the present invention relates to interfacing with communication networks requiring a compatible line termination for terminal equipment interfacing with a hosting network.
2. The Prior State of the Art
Modern data transmission devices such as computers are increasingly used in applications that require exchange of data over a communication network. Popular networks for use in propagating data include well established telephone networks. Such networks were originally designed to accommodate, and therefore propagate, the transmission of voice conversations which include very specific bands of frequencies. In order to facilitate propagation of such bands of frequencies across a network, the hosting telephone network specifies a nominal characteristic impedance with which any attaching or associating terminal equipment, such as a modem, must match or approximate in order to facilitate discernable communication therebetween.
While the popularity of data exchange devices, such as a computer, are well known, computers cannot directly interconnect with a telephone network without modulating the digital data into frequencies and waveforms compatible for propagation across the telephone network. Terminating equipment, such as modems, are specifically designed for this purpose. In order to exchange and transfer data over transmission lines, the terminating impedance of the terminating equipment should equal the characteristic impedance of the transmission line. In order to meet this requirement, the telephone network providers specify a nominal characteristic impedance which roughly approximates the actualimpedance of the transmission lines to which terminal equipment, such as modems, are connected.
Any generally specified characteristic impedance can only be an approximation of the actual characteristic impedance. This results from such variables as: the variations in the length of the transmission lines to the terminal equipment from the central office; various wiring topologies within an intermediary installation such as a series of parallel transmission lines within a business or other structure; and intrinsic variations in the transmission lines themselves. The actual characteristic impedance presented by the telephone network is difficult to precisely match and is usually only approximated.
The ability to precisely match the characteristic impedance of a transmission line has several benefits. First, if the characteristic impedance is matched by the terminal equipment, then the system is able to transfer the maximum amount of power or signal energy. This is important in transmission line applications because of the noise present in the transmission lines. The transfer of maximum power maximizes the signal to noise ratio which enables the data transfer rate to also be maximized. Second, signal reflections can be avoided. When a terminal equipment capable of sending and receiving signals over a transmission line sends a signal when the characteristic impedance is not matched, the sent signal is reflected and becomes part of the signal received by the terminal equipment. This reflected signal can corrupt the original receive data signal and must therefore be removed before the receive signal can be interpreted. A terminal equipment having an impedance that matches the actual impedance of the network can avoid this problem.
The problems associated with signal reflections have been handled in a variety of ways. One method is to employ an echo cancellation circuit. An echo cancellation circuit may employ digital signal processing to cancel the echo, but echo cancellation algorithms are computationally intense. Further, it is very difficult to remove only the echo without affecting the original signal because of the phase shifts attributable to the mismatched impedances. In sum, the necessity of an echo cancellation circuit; whether it be digital or analog, coupled with the mismatched impedances can lead to reduced transmission capacity as well as data errors.
Another method for matching the characteristic impedance of a transmission line is to terminate the transmission line by switching better matched impedances through relays or field effect transistor (FET) switches. This method has several disadvantages. First, control circuitry must be employed to control the relays and switches, which is not a trivial task because of the high voltages which may be present on many transmission lines. Because of the high voltages, the components used for the switches and relays can be large and expensive and must be rated to withstand the high voltages which can be present on a transmission line such as a telephone network.
The need for these types of relays and switches presents an additional problem for persons using terminal equipment such as a PCMCIA modem. The high voltage relays and FET switches, in addition to being costly, consume a large part of the surface area of the printed circuit board (PCB) disposed within the PCMCIA cards and, with regard to PCMCIA cards, surface area is limited and therefore critical.
It would therefore be an improvement to provide a system to substantially, if not precisely, match the actual or nominal characteristic impedance of a transmission line.
It is an object of one embodiment of the present invention to provide a circuit for substantially matching the characteristic impedance of a transmission line.
It is yet another object of one embodiment of the present invention to provide a circuit for substantially matching a characteristic impedance of a network by reflecting a referenced impedance.
It is yet a further object of one embodiment of the present invention to provide a circuit for terminating a network with a scaled impedance such that the reflected impedance substantially matches the characteristic impedance of the network.
A system and circuit are presented which terminate a transmission line with a reflected impedance substantially equal to the characteristic impedance of the transmission line. Transmission lines, such as those found in telephone networks, often have high potentials or voltages. In order for terminal equipment to effectively communicate and send data over transmission lines, the terminal equipment must substantially match the characteristic impedance of the transmission line. However, the high voltages present on many transmission lines make the task of matching the characteristic impedance difficult and costly because the relays and switches required must be rated for high voltages. Also the relays and switches must be triggered at high voltages, which of itself is a difficult task.
The first step of the present invention is to attenuate the transmission line voltage to a level where inexpensive circuit components may be used. In order for the impedance of the terminal equipment to match the characteristic impedance of the transmission line, the present invention uses analog circuit components to generate a current having a value of one over the characteristic impedance of the transmission line. After the line voltage is attenuated, the attenuated voltage is placed across a scaled characteristic impedance.
Both the attenuation of the line voltage and the scaled characteristic impedance are related by a factor Because the line voltage is attenuated and the impedance is scaled, the line current remains unchanged. Thus, the terminating impedance, from the transmission line""s point of view, is the same as the characteristic impedance. In other words, the attenuated line voltage is placed across a scaled impedance to produce a reflected impedance substantially equal to the characteristic impedance of the transmission lines.
The present invention can be an integral part of the terminal equipment, or it can be a separate circuit. Because the actual characteristic impedance of a transmission line may vary, the present invention is capable of connecting with a plurality of scaled impedances to insure that the actual impedance is matched as perfectly as possible.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.